A CT image contrast is largely related to an X-ray source energy spectrum distribution used for scanning. The traditional CT uses a ray source with an energy spectrum distribution for imaging. Sometimes, information ambiguity may occur, which results in two different materials being completely the same on a CT image. The dual-energy CT uses two energy spectrums with different distributions for imaging an object, which can eliminate the information ambiguity in a single energy spectrum. The dual-energy X-ray CT imaging technology can utilize difference between attenuations of a material at different energy levels to obtain distribution information about multiple physical characteristic parameters of an object, for example, an electron density distribution, an equivalent atomic number distribution, and single-energy attenuation images at multiple energy levels. Thus, the dual-energy X-ray CT can be used for calibration of ray hardening of the traditional CT, acquisition of a clinical energy spectrum image with a high contrast, detection of particular and dangerous goods in industry and safety inspection and so on. Compared with the traditional X-ray CT imaging technology, breakthroughs of the dual-energy CT in its imaging function are of great significance in applications such as a medical diagnosis technology, lossless detection and safety inspection etc., and thus attract more and more attention in recent years. In addition, the dual-energy X-ray CT reconstruction method is a search hotspot currently.
Currently, there are three methods for dual-energy CT reconstruction as follows: (1) a post-processing method, in which attenuation coefficient distribution images are reconstructed from low-energy data and high-energy data respectively, then synthesis calculation is performed on the two attenuation coefficient distribution images, and thereby, a single-energy image or a distribution image of an energy independent physical quantity (for example, an atomic number, an electron density) can be obtained; (2) a pre-processing method, in which an energy dependent signal and an energy independent signal are parsed from low-energy data and high-energy data (i.e., the so-called dual-energy decomposition), wherein, the parsed signals belong to a projection region, and then, the parsed signals are reconstructed using a traditional CT reconstruction method; and (3) a synthesis iterative method, in which low-energy data and high-energy data are reconstructed directly using an iterative method. At present, the pre-processing method is widely used, because on one hand, the pre-processing method is more accurate than the post-processing method, and can better eliminate an effect of an x-ray broad spectrum; and on the other hand, the pre-processing method has a less calculation amount than the synthesis iterative method.
At present, with respect to the dual-energy decomposition, there are two decomposition methods, i.e., material based decomposition and dual-effect decomposition (for example, with reference to a non-patent document 1). However, in the dual-effect decomposition method, the equivalent atomic number reconstructed image generally has a poor signal-to-noise ratio, and in contrast, the electron density image has a better signal-to-noise ratio (for example, with reference to a non-patent document 2). In addition, in the dual-energy CT reconstruction, a MonteCarlo method or an experiential method can be used for estimating energy spectrum data of the dual-energy CT system, and moreover, a lookup table can also be established (for example, with reference to non-patent documents 2 and 3).
Prior Art Documents    Non-patent document 1: Y. Xing, L. Zhang, X. Duan, J. Cheng, Z. Chen, “A reconstruction method for dual high-energy CT with Mev X-rays,” IEEE Trans Nucl. Sci. vol. 58, no. 2, pp 537-546, 2011;    Non-patent document 2: Guowei Zhang, dual-energy X-ray imaging algorithm and application research [D], Beijing: Engineering Physics at Tsinghua University, 2008; and    Non-patent document 3: G. Zhang, Z. Chen, L. Zhang, and J. Cheng, Exact Reconstruction for Dual Energy Computed Tomography Using an H-L Curve Method, 2006 IEEE Nuclear Science Symposium Conference Record, pp. M14-462, 2006.
In addition, in the dual-energy CT reconstruction, two primary physical characteristic parameters are an equivalent atomic number and an electronic density. Since there is strong unbalance in a process of the dual-energy decomposition, it results in amplification of a noise of the dual-energy CT reconstructed image, especially amplification of a noise of the equivalent atomic number distribution.